A device of this kind is known from patent DE-PS 30 28 813. In order to generate pressure pulses, a choker valve is installed in the drill pipe in such a manner that the flushing liquid flows towards and around it. All the attachments necessary to drive the choker valve, plus the choker valve itself, constitute an individual structural part of the equipment. Power to operate the choker valve is supplied via channels that run through the cross-pieces which connect the choker valve to the wall of the casing pipe. The cross-pieces lie in the flushing liquid flow. In the case of a drill pipe, such as is known from patent DE-PS 21 61 353, it is usually the flushing liquid which drives the drill bit, via a turbine. Since, however, the flushing liquid is mixed with drillings, the cross-pieces and the turbine are exposed to the very abrasive flow action which quickly leads to their destruction. If the use of a turbine is dispensed with, i.e. that the drill pipe itself is used to drive the core bit, then the drive units for the choker valve, as well as the transmission elements for the signals from the measuring devices to the choker valve, are exposed to heavy stress due to the drilling activity and the rotary motion.
At the present state of technology, a version of a rotating drill pipe is already known, where--in the area of the core barrel--a rotatable inner pipe has been installed in a stationary outer pipe. This part of the drill pipe is also called a target drill rod. Generally speaking, a target drill rod is a casing pipe that is installed in the string of drill pipes, and which receives and transmits the measuring data which originate from the measuring devices and monitors in the target drill rod. Whereas the measuring data give information about the course of the drill hole, i.e. about possible deviations from a desired direction of the drilled well, the monitors deliver measuring data which make it possible to check the functions of the various devices of such a targeet drill rod. Furthermore, the monitors are provided with a device which allows the correction of the drill hole. As a rule, such a device consists of several pivotable control bars, installed on the outer pipe, and which are supported by the joints of the well. They are individually adjustable by means of hydraulically loadable cylinders, and serve to correct the direction of the drill pipe. Such a target drill rod is known, for instance, from patent DE-OS 30 00 239.2. Usually, several, though preferably two bore hole deviation-measuring devices are installed--on vertical measuring planes which are at right angles to each other--in the outer pipe of the target drill rod. These serve to control the hydraulically-loaded adjustment cylinders of the control bars. The data from these bore hole deviation measuring devices not only supplies the input variables of the built-in, automatic control bar adjustment, but are transmitted--by means of the telemetric device--to the control post installed at the well mouth. This telemetric device operates with electric signals which are transmitted via a conductor that is installed either in a trailing cable or in the drill hole itself. The signals, transmitted in this manner, are sufficiently accurate, since they are generated and transmitted by an energy source that is independent of the flushing flow. This energy source supplies the energy for the signal and can also drive the pressure generator, if same generator does not receive its kinetic energy directly from the rotating inner pipe. Even though a battery can also serve as an energy source, it is preferable to have a generator whose rotors are driven by the rotating inner pipe. The conductor connection needed for the transmission of the signals is not very desirable. If placed in the drill pipe, it is difficult to establish and maintain satisfactory physical contact between the casing pipes. If the telemetric device uses a trailing cable, then--though the electrical connection will be satisfactory--it will be exposed to all mechanical as well as other stresses which result from the rotating string of drill pipes, the drill hole joints, and the drill hole flushing.
Furthermore, a target drill rod in the manner of a drill stem and constructed as a rotating casing pipe, is known from patent DE-OS 29 41 102. In this type, the telemetric device is the flushing flow proper that passes through the flushing channel, and a hydraulic converter which transforms the electric signals into pressure impulses of the flushing flow. The thus pressure-modulated flushing flow can be measured at the drill hole exit; and, thus, the pressure impulses can be captured by a receiver which, for forwarding purposes, will convert them into electrical quantities. A poppet valve in the drill stem serves as a transformer for the pressure modulation of the flushing flow as well as for throttling the flushing flow. The poppet valve is activated by a built-in, self-contained hydraulic cycle. The control of the hydraulic means is effected with the aid of a solenoid valve that is loaded with the electric deviation data.
Such a telemetric device presupposes an axial location of the poppet valve, i.e. that the valve body be concentrically located in a flushing channel that leads the flushing flow past the poppet valve, behind the restrictor. On the one hand, this results in a space problem when the casing pipe in which the flushing channel runs has relatively thin walls. This is especially the case when the inner pipe of a target drill rod has a stationary outer pipe. On the other hand, a stationary outer pipe is not feasible when casing pipes with walls of an adequate thickness are used. This requires that the power generator be put in the rotating casing pipe and, thus, would need a flushing-flow-driven turbine for driving the generator. Due to the pressure-modulated flushing flow and other quantities which influence it, this turbine causes faulty formation of the electric signals that are to be transmitted. The result is that if the pressure signals are produced and transmitted by means of the known device, these signals are marked by an irregular, or at least slow pressure rise and drop. This is detrimental since it not only makes difficult the perception of the pressure signals but it also keeps the signal frequency low and, thus, lessens the accuracy of the data transmitted in this manner.